In modern projection exposure apparatuses, electromagnetic radiation in the extremely short-wavelength range, so called EUV radiation, is used for producing very fine structures on semiconductor wafers. Typically, this radiation is produced in a plasma source, subsequently conditioned by way of an illumination system and initially used for illuminating a mask, a so-called reticle, wherein the structures on the reticle are imaged, in a reduced fashion, as a rule, on a semiconductor substrate, the so-called wafer, by way of a projection optical unit. Conventional transmissive or refractive optical elements, such as lens elements, for example, are non-transparent to the aforementioned, extremely short-wavelength radiation, and so reflective elements, so called multi-layer mirrors, are used, as a rule, for conditioning the radiation in the illumination system and for imaging the reticle on the wafer. These multi-layer mirrors are often mounted in an encapsulated partial volume, a so-called mini-environment. There usually is a high vacuum in the aforementioned mini-environment, said high vacuum being purged in regions by hydrogen as a purge gas for the purposes of reducing contamination. However, the action of the EUV radiation, which is used for imaging purposes, on the gaseous hydrogen present in the system leads, in part, to unwanted side effects. The EUV radiation, on account of its high energy, has an ionizing effect on the hydrogen, and so hydrogen ions or a highly diluted plasma arise(s) on account of the action of the EUV radiation on the purge gas. Here, the presence of the plasma has different effects on the optical elements situated in the mini-environments and, in particular, on the surfaces of the multi-layer mirrors. Firstly, the plasma exhibits a certain cleaning effect, which is certainly desired, and so a comparatively high reflectivity of the employed mirror surfaces is maintained as a result of the presence of the plasma. On the other hand, however, an effect that may, for example, occur is that arising hydrogen ions penetrate into the reflective layers of the employed mirrors, recombine there to form H2 and lead to the formation of bubbles or to local splitting of coatings of the mirrors on account of the increased volume requirement.
Thus, it is desirable to influence the plasma spontaneously arising in the mini-environments during the operation of an EUV projection exposure apparatus in such a way that, firstly, the damaging effect of the plasma is reduced and, secondly, the cleaning effect of the plasma is optimized. By way of example, the German laid-open application DE 10 2013 209 442 A1 has disclosed concepts for deflecting plasma constituents from optical or electronic components of an EUV projection exposure apparatus.